This invention pertains generally to methods and apparatus for monitoring nuclear reactors, and more particularly, to such methods and apparatus that employ parameters that are monitored exterior of the reactor core.
Generally, nuclear reactors contain a reactive region commonly referred to as the core, in which sustained fission reactions occur to generate heat. The core includes a plurality of elongated fuel rods comprising fissile material, positioned in assemblies and arranged in a prescribed geometry governed by the physics of the nuclear reaction. Neutrons bombarding the fissile material promote the fissionable reaction which, in turn, releases additional neutrons to maintain a sustained process. The heat generated in the core is carried away by a cooling medium, which circulates among the fuel assemblies and is conveyed to heat exchangers which in turn produce steam for the production of electricity.
Commonly, in pressurized water reactors, a neutron absorbing element is included within the cooling medium (which also functions as a moderator) in controlled variable concentrations to modify the reactivity, and thus the heat generated within the core, when required. In addition, control rods are interspersed among the fuel assemblies, longitudinally movable axially within the core, to control the core's reactivity and thus its power output. There are three types of control rods that are employed for various purposes. Full length rods, which extend in length to at least the axial height of the core, are normally employed for reactivity control. Part length control rods, which have an axial length substantially less than the height of the core, are normally used for axial power distribution control. In addition, reactor shut-down control rods are provided for ceasing the sustained fissionable reaction within the core and shutting down the reactor. The part length rods and full length control rods are arranged to be incrementally movable into and out of the core to obtain the degree of control desired.
As a byproduct of the fissionable reaction, through a process of beta decay of radioactive iodine, xenon is created. Xenon has the property of having a large neutron absroption cross-section and, therefore, has a significant effect on the power distribution within the core and reactivity control. While the other forms of reactivity management are directly responsive to control, the xenon concentration within the core creates serious problems in reactor control in that it exhibits a relatively long decay period and requires up to at least 20 hours after a power change to reach a steady state value.
While the radial power distribution of the core is fairly predictable, due to the prescribed arrangement of fuel assemblies and the positioning of control rods which are symmetrically situated radially throughout the core, the axial power distribution can vary greatly during reactor operation. Core axial power distribution has created many problems throughout the history of reactor operation for many reasons. Normally, coolant flow through the fuel assemblies is directed from a lower portion of the core to the upper core regions, resulting in a temperature gradient axially along the core. Changes in the rate of the fissionable reaction, which is temperature dependent, will thus vary the axial power distribution. Secondly, the axial variation in the power distribution varies the xenon axial distribution, which further accentuates the variations in power axially along the core. This can lead to a xenon induced axial power distribution oscillation which can, late in core life, be unstable without corrective operator intervention. Thirdly, insertion of the control rods from the top of the core, without proper consideration of the past operating history of the reactor, can worsen the axial power peaking.
The change in reactor power core output which is required to accommodate a change in electrical output of an electrical generating plant is commonly referred to as load follow. One load follow control program currently recommended by reactor vendors utilizes the movement of the full length control rods for power level increases and decreases and the part length control rods to control xenon induced spacial axial power oscillations and shape the axial power profile. Changes in reactivity associated with changes in the xenon concentration are generally compensated for by corresponding changes in the concentration of the neutron absorbing element within the core coolant or moderator. In this mode of operation, the part length rods are moved to maintain the axial offset within some required band, typically about plus or minus 15%. The axial offset is a useful parameter for measuring the axial power distribution and is defined as: ##EQU1## where P.sub.t and P.sub.b denote the fraction of power generated in the top half and the bottom half of the core, respectively, as measured generally by two section axially aligned ex-core detector assemblies positioned around the periphery of the reactor. No effort is made to maintain the inherent core axial power profile aside from maintaining the axial offset within the required band. The part length rods are moved to minimize and reduce the axial offset independent of the previously established steady state axial offset. This process induces a constant fluctuation of the axial offset during sustained load follow operation, which results in a number of undesirable operating conditions. For one thing, axial power pinching, which is a large, axially centered power peak, can occur with a low or zero axial offset. Such power peaks result in a reactor power penalty which requires the reactor to be operated at a reduced level so that such peaks do not exceed conservative specified magnitudes. The conservative limitations are imposed due to the inadequacies of present ex-core maintaining systems which do not have the capability of identifying the power level in the center of the core. Secondly, severe changes can occur in the axial power profile of a transient nature during large load changes due to the heavy insertion of control rods at reduced power levels. Thirdly, large xenon transients occur upon coming back to power, resulting in occurrences such as axial power oscillations. Fourthly, improper part length rod usage can produce severe axial power distribution which aren't readily identified by present ex-core detector systems. Fifthly, increased hot channel factors result (which are hot spots which occur within the cooling channels among the fuel assemblies) and require reduction in the power rating of the reactor to accommodate the severe transients and/or adverse power profiles. Finally, no protection currently exists against severe axial pinching with small axial offsets.
Due to the many adverse operating conditions experienced in operating a nuclear reactor during load follow, many reactor vendors recommend operating the reactor at a constant power output without a load follow capability. This lack of versatility in plant operation limits the utility of reactors and requires that fossil-fuel electric generating plants be sustained to maintain the differences in capacity required with load changes.
To establish an effective load follow capability, a substantially constant axial power profile will have to be maintained throughout load operation. Application Ser. No. 501,569, filed Aug. 29, 1974 addresses this problem by maintaining a substantially symmetric xenon axial profile. However, to effectively maintain a substantially constant axial flux profile, a monitoring system is required that has the capability of substantially reconstructing the flux axial pattern within the core so that variations therein can be accurately compensated for before a xenon maldistribution is effected.
While the in-core flux monitoring system described in U.S. Pat. No. 3,932,211, issued Jan. 13, 1976 is capable of providing an accurate picture of the axial flux profile, employing in-core detectors, such detectors being subject to the high flux environment of the core, are susceptible to early burn-out if employed consistently for this purpose. Generally, such detectors are employed to provide flux maps at start-up of the reactor or periodically thereafter to calibrate the ex-core detectors or, as described in the afore-cited application, after large control rod movements are initiated. However, an effective core monitoring system will require a continuous core map of the axial flux profile to be an effective tool. The ex-core detectors have been employed in the past for this purpose because they have been demonstrated to be more reliable, being in a lower flux, dry, low temperature, lower relative pressure environment outside of the pressure vessel.
Accordingly, a new flux monitoring system is desired that can provide an accurate picture of the axial flux profile over the entire height of the core. Further, such a system is desired that has a reliability comparable to that of the ex-core detectors with the degree of accuracy and definition obtained from in-core movable detector systems.